1. Field of the Invention
The invention relates to liquid crystal display (LCD) devices, and more particularly to fringe field switching mode liquid crystal display (FFS-LCD) devices.
2. Description of the Related Art
Liquid crystal display (LCD) devices possess the advantages of small size, light weight and low power consumption thus, they offer enhanced portability and applicability to a wide variety of electronic and communication devices including notebook computers, personal digital assistants (PDA), mobile phones and similar. Critical features for large-scale monitors and high-end TV applications include fast response, high contrast ratio, high transparency, and wide viewing angle with no gray scale inversion.
Fringe field switching liquid crystal displays (FFS-LCD) meet the described high quality display feature requirements, and solve the viewing angle problems by orienting the liquid crystal molecules to be parallel with a substrate. Moreover, FFS-LCDs have high a aperture ratio and transmittance.
LCDs typically present images by mixing the three primary colors, red, green and blue. In addition to the three primary colors, a white is also provided to increase transmissive or reflective brightness and thus reduce power consumption. Mixed RGBW LCDs, however, suffer from some drawbacks.
FIG. 1 illustrates a conventional RGBW LCD comprising a first substrate 100, such as an array substrate. A plurality of thin film transistors (TFTs), each comprising a gate, a source and a drain, is formed on the first substrate 100. In this figure, the TFTs and/or other active devices are simplified as an active layer 102. A lower alignment layer 104 is formed on the active layer 102.
A second substrate 106 is opposite the first substrate 100. Red resist patterns R′, green resist patterns G′ and blue resist patterns B′ are formed on an inner side of the second substrate 106 each comprising openings 110. The red resist patterns R′, the green resist patterns G′ and the blue resist patterns B′ can be formed by photolithography. The openings are provided at locations for forming white resist patterns. After forming the R′, G′, B′ resist patterns, a coating layer 112 is blanketly deposited to fill the opening 110, and thereby form the transparent white resist patterns W′. Next, an upper alignment layer 114 is formed on the coating layer 112. Liquid crystals are injected into a space between the upper alignment layer 114 and the lower alignment layer 104 to form a liquid crystal layer 116.
Because the white resist pattern W′ is formed by filling gaps in the coating layer 112, non-uniform cell gaps occur. For example, a cell gap d1 corresponding to the white resist pattern W′ is larger than another cell gap d2 corresponding to the red resist pattern R′, the green resist pattern G′, and/or the blue resist pattern B′. Non-uniform cell gaps, however, present non-uniform T-V (transmittance versus voltage) curves. As shown in FIG. 2, different cell gaps, such as 3.5 μm, 4.0 μm, and 4.3 μm, achieve different T-V curves. Accordingly, some optical properties such as contrast ratio and viewing angles may deteriorate in LCDs.
FIG. 3 shows a cross section of a conventional fringe field switching liquid crystal display (FFS-LCD) device 300, which comprises a first substrate 302 and an opposite second substrate 304. A common electrode 308 is formed on the first substrate. An insulating layer 312 is formed on the common electrode 308. A plurality of pixel electrodes 310 are formed on the insulating layer 312. A lower alignment layer 314 is formed to cover the insulating layer 312 and the pixel electrodes 310. A color filter layer 316 and an upper alignment layer 318 are formed on the inner surface of the second substrate 304. A liquid crystal layer 306 is interposed between the upper and the lower alignment layers. The FFS-LCD device achieves wider viewing angle by orienting the liquid crystal molecules 320 of the liquid crystal layer 306 to be parallel with the first and second substrates 302 and 304 using an electric field 322 between the pixel electrodes 310 and the common electrode 308, and further has advantages of high aperture ratio and transmittance. The conventional FFS-LCD device, however, still suffers from non-uniform T-V curve and related issue when incorporating mixed RGBW technology.